John McCain and Lindsey O. Graham, both Republicans, represent Arizona and South Carolina in the Senate, respectively. Joseph I. Lieberman, an independent, represents Connecticut in the Senate.

As 2012 draws to a close, Syria is descending into hell. At least 40,000 people, and likely many more, have been killed, while millions have been forced to flee their homes. Over the past 12 months, Bashar al-Assad has steadily unleashed ever-greater military firepower in response to what began as peaceful protests by the Syrian people. Starting with tanks and heavy artillery in February, the Syrian regime escalated over the summer to using attack helicopters and fighter jets. In recent weeks, it has begun firing Scud missiles at its own population.

The world has failed to stop this slaughter. President Obama has declared that his “red line” is Assad’s use of chemical weapons. Many Syrians, however, have told us that they see the U.S. red line as a green light for Assad to use all other weapons of war to massacre them with impunity. Many of those weapons continue to be supplied directly by Iran.

Despite the U.S. government’s warnings, Assad has reportedly taken steps in recent weeks to prepare chemical weapons for use against his people. From everything we know about Assad’s regime, and considering that he has methodically escalated this conflict using nearly every other weapon in his inventory, does anyone really believe that this man is incapable of using chemical weapons?

Syria’s descent into hell poses an increasing threat to its neighbors. Turkey, Lebanon, Iraq, Jordan and Israel all face rising risks of instability. The longer this war grinds on, the greater the chance it could ignite a wider sectarian conflict.

For months we have argued — including on this page — that the United States, together with our allies in Europe and the Middle East, must do more to stop the killing in Syria and to provide help to moderate forces among the opposition. Specifically, we have advocated providing weapons directly to vetted rebel groups and establishing a no-fly zone over part of Syria. Neither course would require putting U.S. troops on the ground or acting alone. Key allies have made clear again and again their hope for stronger American leadership and their frustration that the United States has been sitting on the sidelines.

Most distressing of all are the swiftly deteriorating humanitarian conditions in Syria. While rejecting calls to provide weapons or establish a limited no-fly zone, the Obama administration has emphasized the aid that it has committed to the Syrian people. We fear, however, that those efforts are also failing.

According to U.S. and European officials and experts, as much as 70 percent of the foreign assistance being sent to Syria ends up in regime-controlled areas. Recent visitors to Aleppo have told us they saw no sign of U.S. aid there, nor were local Syrians aware of any American assistance. As a result, people in the opposition-held north of Syria are starving, freezing and dying from disease because of shortages of food, fuel and medical supplies.

This failure to get American humanitarian assistance to the Syrian people has not only worsened the humanitarian crisis but has also created opportunities for extremist groups to provide relief services and thereby win even greater support from the Syrian people. To many, these extremists appear to be the only ones stepping in to help Syrians in the fight. Meanwhile, moderates in the Syrian opposition are being discredited and undercut by our lack of support — including the newly established Syrian opposition coalition, whose formation last month was made possible in part by U.S. diplomacy.

While recent regime defections and battlefield setbacks suggest that Assad’s hold on power is deteriorating, this conflict could grind on for some time, at an awful and escalating cost to Syria’s people, its neighbors and U.S. interests and prestige. It is not too late to avert a strategic and moral calamity in Syria, but doing so requires bold and decisive U.S. leadership that needs to come directly from President Obama.

The United States must rally our allies to channel assistance to the newly established Syrian opposition council for distribution in the rebel-held areas. We must provide weapons and other lethal assistance to the opposition military command. And we must impose a no-fly zone in some areas of Syria, to include using the U.S. Patriot missile batteries en route to Turkey, to protect people in northern Syria from Assad’s aerial attacks.

If we remain on the current course, future historians are likely to record the slaughter of innocent Syrians, and the resulting harm done to America’s national interests and moral standing, as a shameful failure of U.S. leadership and one of the darker chapters in our history. That should unsettle us all as we pray for peace and goodwill this holiday season.

Comments our editors find particularly useful or relevant are displayed in Top Comments, as are comments by users with these badges: . Replies to those posts appear here, as well as posts by staff writers.

To pause and restart automatic updates, click "Live" or "Paused". If paused, you'll be notified of the number of additional comments that have come in.

Comments our editors find particularly useful or relevant are displayed in Top Comments, as are comments by users with these badges: . Replies to those posts appear here, as well as posts by staff writers.